Summary:
PhoR is the sensor kinase of the PhoRB two component signal transduction pathway. PhoR indirectly senses and responds to variations in the level of extracellular inorganic phosphate (Pi) by phosphorylating or dephosphorylating its cognate response regulator PhoB.

PhoR is known to be a inner membrane protein comprised of five domains including an N-terminal transmembrane domain, a dimerisation and histidine phosphoacceptor domain and a C-terminal kinase domain [Yamada90a, Hsieh10a]. PhoR is an atypical sensor kinase as it does not contain a large periplasmic domain [Scholten93]. The signal sensing and kinase domains of PhoR are located in the cytoplasm [Scholten93].

Under limiting environmental Pi, PhoR is believed to be in an active signalling conformation resulting in the phosphorylation and activation of PhoB and subsequent induction of the Pho regulon. Activation under limiting environmental Pi is considered to be the default state. Excess levels of Pi (>4uM) result in an inhibited form of PhoR which interferes with phosphorylation of PhoB and prevents its activity as a transcription factor [Makino89, Hsieh10a]. The Pst transporter PstSCAB and the chaperone like protein PhoU are both necessary for inhibition of the PhoRB system [Cox88, Wanner93]. Accordingly an inhibition complex of PhoR, PhoU and the Pst transporter (PstSCAB) has been proposed but the nature of the interaction between these proteins is not clear [Hsieh10a].

Based on DNA microarray analysis, it has been shown that PhoBR and PhoB are responsible for upregulation of phosphonate and glycerol phosphate metabolism and the high-affinity phosphate transport system, respectively, thus showing the complex regulation by the PhoR-PhoB two-component regulatory system [Baek07].